Along with the advancement of IT and the spreading of the Internet, amount of data handled by computer systems in companies and the like are continuing to increase. On the other hand, there are high needs to cut down costs related to the IT system, and there are demands for a high-performance and inexpensive system.
In a storage device, storage media used in the storage device occupy a large portion of the cost. Therefore, an important problem is to cut down the costs of the storage media. In addition to a method of adopting inexpensive (low bit cost) storage media, a method of compressing stored data to enable a greater amount of data to be stored in a storage media can be adopted to cut down the costs of the storage media. When data is stored in a compressed state to the storage media of the storage device, it is important from the viewpoint of user-friendliness that host computers and applications accessing the storage device are not conscious of the fact that data is stored in a compressed state, that is, data compression is performed transparently. It is also practically important to prevent deterioration of the access performance as much as possible. For example, Patent Literature 1 discloses a system for accessing necessary data, where data (uncompressed data) is divided into a plurality of units and compressed per unit, the respective compressed units are stored in an LU (Logical Unit), and only the necessary unit is read without reading the whole LU when reading data.